This invention relates to monolithic transistor circuits, and in particular to providing a large value resistance across the emitter-base junction of a transistor in said circuit.
There are many presently available circuits employing at least two stages of current amplification. For example, Darlington circuits are well known for use as amplifiers and switches. Such circuits employ two transistors which are cascaded by connecting the emitter of the input transistor to the base of the output transistor and interconnecting the collectors. It is most advantageous to fabricate such a circuit as a monolithic device where the transistors are formed in a single semiconductor wafer with a common collector region. Various structures of this type have been proposed (see, for example, U.S. Pat. Nos. 3,177,414 issued to Kurosawa, 3,596,150 issued to Berthold, 3,755,722 issued to Harland, Jr., and 3,995,308 issued to Weinstein). The primary advantage of such circuits is the resulting gain, which is the product of the gains of the individual transistors.
One of the inherent problems associated with such a circuit is the presence of leakage currents across the collector-base junction of the input transistor which tend to turn on the transistor when no input information is supplied. Due to the coupling of the transistors, this leakage current is amplified in the second transistor. The usual approach to this problem is to provide resistors across the emitter-base junctions of the transistors in order to bleed off the leakage current. Such conventional bleed resistors are effective for most device applications.
For high voltage devices, however, it is often not practical to produce elements of sufficient resistance on a small discrete chip. For example, a Darlington switch with a phototransistor as the input and operating at approximately 300 volts can produce leakage currents of the order of 10 nanoamps which are amplified to of the order of 10 microamps. Such high leakage currents are due, in part, to the high gain needed for optically triggered devices and the need for a large area collector-base junction for light collection. In addition, it is desired to turn on the device when, for example, 40 microamps of current is generated in the phototransistor. What is required in such devices is a resistance of the order of megohms across the emitter-base junction of the phototransistor and a resistance of the order of kilohms across the output transistor. While several proposals exist for providing integral resistors of the order of a few kilohms in Darlington circuits (see, for example, U.S. Pat. No. 3,596,150 issued to Berthold, and U.S. Pat. No. 3,755,722 issued to Harland), there does not appear to be any practical method for providing integral resistors of higher orders of magnitude which are required for the type of high voltage devices described herein.
The invention is therefore directed to the problem of providing a high voltage monolithic transistor circuit with integral means for bleeding off leakage currents.
A further problem which is often encountered in such circuits results from the fact that the bonding pad to the emitter metallization of the output transistor is often placed over the area of the base of that transistor. If the insulation between the metal and the base region is not free from cracks, a short will develop. Consequently, a further feature of the invention in accordance with one embodiment is directed to eliminating this shorting problem.